partial.php

<?php
//https://codex.wordpress.org/WPMU_Functions/switch_to_blog


interface iPartial
{
    public function get_site_path($blog_id);
    public function get_main_slug($slug);
}

class Partial implements iPartial
{
    public function get_site_path($blog_id) {
        $self = get_blog_details($blog_id)->path;
        if ($self != '/') {
            return false;
        } else {
            return true;
        }
    }
    public function get_main_slug($slug) {
        //if site url has no path added
        //main site blog id would == 1
        $blog_id = get_current_blog_id();
        if (get_site_path($blog_id)) {
            switch_to_blog( 1 );
                //call private partial slug only after switching blogs
                sl9_private_partial($slug);
            //go back to main site
            restore_current_blog();
        }
        
    }
}